Despite my tiredness, I always try to psych myself up for working out throughout the day. I think of it as "me time" and try to go over why I'm doing it and how healthy it is. Goods tunes on my mp3 player sure helps too.
Like mkculs suggested, drink some cold water. Sometimes I'll even run to the bathroom and splash my face a little bit (or if you have makeup on, use a paper towel). I always eat a snack in the afternoon to keep me going and it's also the fuel I use for the gym. Usually oatmeal or something, but sometimes fruit. Food gets my blood sugar back up.
Finally, move around your office if possible. When you feel that after lunch lag set in, go to the bathroom, chat with a coworker, stretch, go to the breakroom, hang by a fax machine and look like you're doing something important, lol. Anything to get you away from your desk, if possible. I simply go crazy if I can't get up from my desk about once an hour.